When they carefully plotted out the data, the researchers found that the human frontal lobes were just the size you would expect them to be for our total brain size.

"We know that the human brain has expanded rapidly overall, but we've shown that the assumption that this was driven by the frontal lobes expanding particularly fast is untrue," says Barton.

Their analysis found the frontal cortex did not evolve especially fast relative to other brain regions after the evolutionary lineage of humans and chimpanzees diverged.

Size is relative

It has been "a long-running notion" that the frontal lobes expanded faster than other regions of the brain as it evolved, says Dr Klaus Stiefel of the University of Western Sydney.

But Stiefel says the study is "technically very well done" and clearly shows human frontal lobes are not unusually large relative to the total size of the brain.

The key to the new analysis has been carefully allowing for 'allometry' -- the fact that relationships between one body part and the overall size of the animal may not be to scale.

"Think about a dog-sized elephant, says Stiefel.

"If you scaled it exactly, it would have extremely short and stubby legs."

But, he says, this is not how an animal of that size would actually be.

This scaling effect makes it really difficult to compare relative sizes of brain regions in human brains with say a macaque or marmoset, Steifel says.

In fact, some very odd results can occur if allometry is ignored, says Barton.

"Some mammals have frontal cortex that is a larger proportion of their brains than species that we think of as more advanced - for example some small weasels have relatively larger frontal lobes than some monkeys."

And llamas have proportionately more frontal cortex than macaque monkeys, he says.

Neural networks

Barton and Venditti's study doesn't overturn the idea that specialisation of our frontal lobes may contribute to what makes us human.

But they say the spotlight should be taken off the frontal lobes in isolation.